The present invention relates to thermoplastic resins containing rubbery grafted acrylate rubber particles. More particularly, the present invention relates to acrylate rubber-modified styrene acrylonitrile copolymer resins having improved impact strength weatherability, and surface appearance.
It is previously known in the art to prepare impact-modified thermoplastic resins and, in particular, copolymers of styrene and acrylonitrile, wherein the impact-modifying rubber is a homopolymer or copolymer of an alkyl acrylate having from 2 to about 10 carbon atoms in the alkyl group and having one or more distinct particle sizes. In U.S. Pat. No. 4,442,263, there is disclosed a resin having a large rubber particle size of a volume average particle size from about 5,000 Angstroms to about 8,000 Angstroms, a crosslinker content from about 0.5 percent to about 10 percent by weight, preferably 1 percent to about 5 percent by weight, and grafting levels of about 20 percent to about 60 percent, and preferably 40 percent to about 50 percent. In U.S. Pat. No. 4,224,419, there is disclosed a weatherable resin having both large and small particles of acrylate rubber. The large particle component has a weight (volume) average particle diameter of about 2000 .ANG. to 5,000 .ANG. and a graft level of 20 percent to 40 percent. The small particle component has a weight (volume) average particle diameter of about 500 .ANG. to 1500 .ANG. and a graft level of 25 percent to 45 percent. The amount of crosslinker in both the large and small rubber particles is 0.5 percent to 10.0 percent (preferably from 1.0 percent to 5.0 percent). It was not taught that there was any advantage in having significantly higher levels of crosslinking monomer in the small particle than in the large particle or that very low levels of crosslinker in the large particle (less than 0.5 percent) provided improved rubber toughened vinyl aromatic resins. Additional examples of acrylate rubber-modified styrene/acrylonitrile resins are disclosed in U.S. Pat. Nos. 3,691,260; 3,793,402; 3,992,485; 4,082,895; 4,341,883; 3,830,878; 4,433,102; and 4,224,419. For the teachings contained therein, the previously mentioned patents are incorporated herein by reference thereto.
Previously known resins including resins containing more than one rubber particle size have failed to achieve optimum properties. In particular, previously known resins have proven deficient in one or more physical properties such as impact strength, weatherability, or surface appearance.
It would be desirable to provide an acrylate rubber-modified styrene/acrylonitrile resin having an improved balance of properties.